The following discussion of the prior art is not to be regarded as an admission that what is discussed is common general knowledge in this field.
Plastic film bags have become a common packaging method for food and other products, including liquid food products. Such bags offer substantial benefits over traditional rigid containers such as bottles, cans, pails, drums or tanks. The bags are usually fitted with spouts and caps. Fitments can be used for filling product into the bag, pumping product out of the bag, or both.
The dispensing systems for such bags commonly are of two types. A first type employs a valve arrangement and is gravity driven (e.g. a wine cask), whilst a second type uses a pump system (e.g. a peristaltic pump) with or without a valve arrangement. There are numerous disadvantages associated with prior art dispensing systems including:                1. Dispensing accurate doses of liquid at a relatively fast rate is difficult. This is particularly the case if the liquid is viscous and not ‘runny’. The speed of peristaltic pumps is limited because the flexible tube needs to return to its original shape between pumps (i.e. the pumps need to be spaced in time to allow this). Peristaltic pumps are relatively power inefficient and thus can be relatively costly to operate. Furthermore, peristaltic pump tubes wear out regularly and so need to be replaced. The accuracy of peristaltic pumps is also limited since the rate of volume throughput is a function of hydraulic pressure (which can vary depending on a range of factors). It is acknowledged that slowly dispensing liquid to improve accuracy is possible but this may impact heavily on the commercial feasibility of such a system. Alternatively, machines that can dispense viscous liquid at a fast rate tend to be complex and expensive.        2. In the case of systems incorporating a pump, liquid can remain in the pump and/or the pump may not be strong enough to suck the last portions of liquid out of the bag. Failure to substantially completely empty the bag results in wastage of liquid, and if the bag is to be reused, the hygiene problem discussed below.        3. They can leave portions of liquid in or on components of the dispensing system. Because of this, the system needs to be regularly cleaned to maintain hygiene and to mitigate the risk of contamination of the liquid dispensed by the system. Even if the liquid receptacle takes a normally closed or sealed form, entry to possible contaminants would be difficult to avoid when the receptacle is refilled. A positive displacement piston type pump is a good example of a type of pump that is generally difficult to clean.        4. They are relatively complex in construction.        
The present invention seeks to provide a liquid dispensing apparatus which will overcome or substantially ameliorate at least one of the deficiencies of the prior art, or to at least provide an alternative.